package com.leetCode.HashDemo;

import java.awt.*;
import java.util.*;
import java.util.List;

public class LetterCombinationsDemo {
    public static void main(String[] args) {
        System.out.println(letterCombinations("23"));
    }

    /**
     * 给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。
     *
     * 给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
     *
     *
     *
     * 示例 1：
     *
     * 输入：digits = "23"
     * 输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
     * 示例 2：
     *
     * 输入：digits = ""
     * 输出：[]
     * 示例 3：
     *
     * 输入：digits = "2"
     * 输出：["a","b","c"]
     * 提示：
     *
     * 0 <= digits.length <= 4
     * digits[i] 是范围 ['2', '9'] 的一个数字。
     * Related Topics
     * 哈希表
     * 字符串
     * 回溯
     */
    public static List<String> letterCombinations(String digits) {
        List<String> combinations = new ArrayList<>();
        if (digits == null || digits.length() <= 0){
            return combinations;
        }
        Map<Character,String> phoneMap = new HashMap<Character,String>(){{
            put('2', "abc");
            put('3', "def");
            put('4', "ghi");
            put('5', "jkl");
            put('6', "mno");
            put('7', "pqrs");
            put('8', "tuv");
            put('9', "wxyz");
        }};
        backTrack(combinations,phoneMap,digits,0,new StringBuffer());
        return combinations;
    }

    //回溯算法
    public static void backTrack(List<String> combinations,Map<Character,String> phoneMap,String digits,int index,StringBuffer combination){
        if (index == digits.length()){
            combinations.add(combination.toString());
        } else {
            char digit = digits.charAt(index);
            String letters = phoneMap.get(digit);
            int lettersCount = letters.length();
            for (int i = 0;i < lettersCount;i++){
                combination.append(letters.charAt(i));
                backTrack(combinations,phoneMap,digits,index + 1,combination);
                combination.deleteCharAt(index);
            }
        }
    }
}
